Seems they've dropped it. I know its not the outright cheapest, but for people wanting the Telstra network (which in many rural areas is superior) without needing a huge amount of calls, the $15 plan with the Rewards discount to $13.50 was a good choice for them. I had many family members on it who don't get great signal on the other networks.
But now it seems that they won't be offering that plan and have yet to replace it. $20 is now the lowest tier.
They dropped both $15 and $45 Recharges according to the email. It will be discontinued from 13 June 2018.
If you want to keep using the $15/45 recharges, you can set up Auto recharge before 13 June and your set to continue to recharge on the old denomination until you change to a new plan. This is according to the email